In response to the last answer, allow me to agree, though I'd twist the answer slightly. I'm not sure that it makes the narrator more reliable, but it does allow us to see the problems with the narrator's tale much easier.

For instance, when Ethan and Mattie have dinner, my favorite paragraph says how she "seemed fuller" and overall, becomes incredibly attractive...just because she had a red bow in her hair. The narrator doesn't take issue with the sudden change in appearance, but merely presenting it to the reader allows us the chance to question how Ethan could see her that much more sexual, just because of a single red ribbon.

Perhaps  it's more reliable because it highlights peculiar passages. I don't wish to misspeak and put words in someone's mouth, but that's plausible.

David Becker 

In addition to the other response, I always point out to my students that this also makes the narrator more reliable. Edith Wharton was a well-educated, articulate, upperclass woman, and Starkfield is a rural town and Ethan Frome, though having some education, is poor and quite inarticulate. Creating a narrator from outside of Starkfield who comes in and pieces together the story of the Fromes allows us to trust the descriptive, articulate tale that is woven within the chapters of the text.